By Ryan Dilley BBC News Online correspondent in New York

New Yorkers may now live in dread of bio-terrorism, but potentially harmful substances already hang in the air over their city - the smoke and dust created by the World Trade Center collapse.

"It was like trying to breathe in the contents of a vacuum cleaner bag," says one of the New York detectives caught in the huge dust cloud thrown up on 11 September as he shows journalists around Ground Zero.

Dust fell like snow as the towers collapsed

Behind him, smoke and steam still rise from the rubble a month on from the tragedy. The shadows of surrounding buildings are stark against a thick haze of dust.

Even half a mile away, the sun peers weakly through the airborne debris. Further afield, an ill wind can still send pedestrians scuttling for refuge from the acrid smell emanating from downtown.

Face masks were once de rigueur for those trying to carry on their normal lives in Lower Manhattan. Now only a few people take the precaution.

Local worker Nana Anokrah has heard assurances that the air is not laden with harmful particles, "but I just don't what's going on," he says through a cotton wool mask. "I guess they've opened up the schools and they wouldn't let the children go back if it wasn't safe."

Asbestos risk

Potentially harmful asbestos was used in the construction of at least a portion of the World Trade Center. The towers' collapse is thought to have launched fibres into the air, particles so small that Mr Anokrah's mask would be hard pressed to keep them out.

Even with closed windows, the dust seems to sneak its way into the house through the cracks

Kathryn Freed

"I suppose it offers a little bit of protection. At least I won't breathe in as much."

Tess O'Dwer sports a far more sophisticated respirator. "My father died of asbestosis [scarring of the lungs by exposure to the fibres]. I know there is no such thing as a 'safe' level of asbestos in the air."

Like many who brave the streets around the financial district, Ms O'Dwer says she has experienced headaches and a persistent sore throat since 11 September.

Although her apartment on nearby West Street has been professionally cleaned since the attack, Ms O'Dwer says the odour inside is still so strong that she will not return home until another clean-up team goes in.

Dust blanketed homes and offices in the area

The greatest fear for local residents and workers is that levels of airborne asbestos are not dropping with the passing days. So-called "spikes" are occurring in the air quality readings being taken by the Environmental Protection Agency (EPA), which exceed the legal level for asbestos content (0.01 fibres per cubic centimetre of air).

Near Ms O'Dwer's home, monitoring equipment registered 0.05 f/cc in the week after the attacks. This level fell, but then surged to more than eight times the safe limit on 29 September.

Clearing the rubble

Kathryn Freed is councillor for the First District, which covers all of Lower Manhattan. She says the sight and smell of the constant dust clouds are "decidedly worrisome" to her constituents.

The sealing of her windows, a professional clean-up and the purchase of air purifiers have not stopped Ms Freed succumbing to the same symptoms which dog many locals. "I have a headache right now," she says.

More than 290,000 tons of rubble have been removed from the site

"Even with closed windows, the dust seems to sneak its way into the house through the cracks. You clean and the next day the white film of dust is back.

"There's no point in using a normal vacuum cleaner. The filters won't catch asbestos and you merely succeed in making the fibres airborne again."

Ms Freed says the work of clearing and transporting the debris from the site is responsible for disturbing and distributing the dust.

"I don't think they are wetting down the debris in the trucks as much as they say they are, because dust still seems to be flowing out as they drive past.

"Perhaps they should be putting the debris in sealed containers. After all, the dock where they load the rubble onto barges is right next to Stuyvesant High School."

'The risk is low'

The EPA, which recently removed sand and top soil from a local playground as a precaution, has been trying to allay fears that asbestos levels in the open air are a hazard to health.

A crane lifts workers above the ruins

Joel Shufro, executive director of the New York Committee for Occupational Safety and Health, says that any exposure to asbestos can theoretically cause cancer.

"There's no threshold limit, although the greater your exposure to the fibres, the greater at risk you are. I'd say that since the fibres are more quickly disbursed in the open air, the risk is low. But it's a bit of a leap to say it is safe."

Mr Shufro says the greatest problem may occur if dust containing asbestos is allowed to settle, only to be disturbed once again and sent into the lungs of those nearby.

"But you have to remember that even before 11 September and even on a good day, there was always asbestos in New York's air."